This invention relates to a tone signal processing device using a digital filter and, more particularly, to realizing rich filter characteristics by preparing filter coefficients by interpolation and further to realizing timewise change of tone color by variably controlling filter characteristics with lapse of time and, still further, to controlling selective supply of filter coefficients in accordance with tone color control information.
An electronic musical instrument realizing timewise change of tone color by timewise changing filter coefficients of a digital filter for controlling a tone signal is disclosed in European Patent Application Publication No. 140,008. In this electronic musical instrument, filter coefficients are stored in a memory in correspondence to a predetermined time frame, filter coefficients corresponding to each time frame are read out from the memory at each time frame and applied to the digital filter whereby filter characteristics are changed frame by frame by changing the filter coefficients frame by frame.
In a case where filter characteristics are to be variably controlled in order to realize tone color control in response to tone color information including key touch and key scaling, filter characteristics have to be prestored in a memory for each realizable filter characteristic.
In the prior art technique in which filter coefficients do not change during one time frame, tone color change is not obtainable when one time frame is relatively long with resulting monotonousness in the tone color. Besides, the tone color change tends to become unnatural due to abrupt change in filter coefficients when the time frame changes. Such problems can be overcome if the time length of one time frame is shortened. This however leads to another problem in that it requires a filter coefficient memory of increased capacity.
Further, in a case where tone color control is to be performed in accordance with tone color control information including key touch and key scaling, if rich tone color change control is desired, multiple sets of filter coefficients must be prestored in a memory and so the capacity of the filter coefficient memory must be increased.
In the filter coefficient memory of the prior art technique, addresses are simply assigned to respective sets of filter coefficients stored therein and one set of filter coefficients is read out from an address corresponding to the tone color control information.
In this type of filter coefficient selection system, however, the relation between address storing filter coefficients and tone color information is a fixed one-to-one relation so that a degree of freedom in the tone color change control is scarce.